Composites Some tips for working on Cirrus composite structures

航空

游客，如果您要查看本帖隐藏内容请回复

航空

16 July 2006 . Aircraft Maintenance Technology . www.AMTonline.com . www.AMTSociety.org
Airframe Technology
Different tools can be useful in visual inspections. Of course, a good flashlight is essential. A good tip to help locate defects such as slight bulges is to shine the flash-light at a low angle across the structure (versus shining it straight on). This angle will make surface defects pop out more.
Coin tap test
A coin tap test is used to determine laminate damage in the composite. Just as with performing a tap test on other com-posite components, the tap test uses acous-tic sounds produced when a small metal object is tapped on the surface. Mechanics are looking for a clear, sharp sound. A dull thud would indicate a void or delamina-tion. Here are a few things to consider when performing a tap test.
Familiarize yourself with the structure you are inspecting. A change in the sound does not necessarily indicate a defect. For example, if tapping different areas of the wing, you would get a slightly differ-ent sound for areas that are sandwiched laminate, non-sandwiched laminate, and transition areas between the two. Corrie Volinkaty, a technical instructor for Cirrus, shares, “You will get slightly different sounds as you transition between differ-ent areas of the aircraft depending on the structure. The thing to remember is that a delamination will not be just a different sound, it will be a dull sound or a thud. Knowing the structure beneath is very helpful when performing tap tests.”
.
When performing a tap test, locate an undamaged area with similar struc-ture to that of the area with suspected damage. Tap the known good struc-ture and use that as a reference when tapping the suspect area.
.
Use a consistent rate and force when tapping the structure.
.
When performing the tap test, mark the outer edge of the damaged area with a Sharpie.

Damage classification
Now that we have used visual and coin tap tests to evaluate the damaged area, we need to classify it. Damage can be classi-fied into four classes: cosmetic, secondary bond, solid laminate, and sandwich struc-ture damage. Always refer to the Cirrus maintenance manual for damage classifica-
Visual inspection tips
The Cirrus maintenance manual offers the following guidelines for per-forming visual inspections:
1.
To enhance familiarization with fuselage construction, examine the
Aircraft Zone Diagrams.

2.
If necessary, remove surface coat around damaged area.
3.
Examine component tactually by running hands over surface of sus-pect repair area to feel for surface imperfections and anomalies.
4.
Paint is generally more brittle than the composite and will crack
before the laminate is damaged.

5.
Cracked paint does not indicate the extent of the damage, only that
damage has occurred.

6.
While dimple and dent damage is similar in appearance to hail damage on a metal surface, delamination and disbanding is more difficult to detect. It is sometimes possible to feel this type of damage by pressing on the area.A disband or delamination may feel soft and movement between the separated layers may be detected.
7.
If possible, use a back light to reveal internal defects and delamina-tions.
8.
Examine exposed laminate for stress whitening.
9.
If possible, the backside of the suspected area should be examined. A borescope can be a helpful tool to examine interior areas.A tip for inspecting interior structure — interior surfaces are usually not paint-ed and damage to glass-fabric structures will show up as a white area.
10.
Use a Sharpie to mark suspect areas found to facilitate coin tap test.

tion. Here are some highlights of the differ-ent classifications.
Cosmetic
Cosmetic damage is non-structural dam-age. It includes minor gouges, scratches, dents, or other defects. Cosmetic defects are defined to the surface protection of the laminate.
Secondary bond damage
This is when disbonding occurs between two pre-cured components. This could be structural or non-structural in nature.
Solid laminate damage
This is structural or non-structural dam-age which extends beneath the surface protection and affects the solid laminate structure underneath. This could be either penetrating or non-penetrating.
Penetrating. This includes fractures and penetration through the laminate.
Non-penetrating. This includes abra-sions, delaminations, surface impact, or gouges through one laminate surface.
Sandwich structure damage
As the name implies, this is damage to composite sandwich structures.
www.AMTSociety.org . www.AMTonline.com . Aircraft Maintenance Technology . July 2006 17
Airframe Technology
Laminate only. This is damage to only one side of the sandwich structure with no core damage. This type of damage to the sand-wich structure can be identified using the solid laminate damage classifications.
Laminate and core. This is when there is damage to one side of the sandwich construction with dam-age to the core. There is no damage to the opposite laminate surface.
Sandwich penetration. This is damage to both sides of sandwich construction. Both laminate sur-faces are punctured and foam core is exposed.
Repair classification
Now that we know what type of damage we have, we need to determine what type of repair we are looking at. Cirrus breaks repairs down into four categories: cosmetic repair, minor repair, major repair, and restricted repair.

Repair Environment
Cirrus offers the following guidelines for set-ting up a proper composite repair environment:
.
The area must be completely enclosed.
.
Temperature and humidity must be con-trolled so that the minimum temperature is 60 F (16 C) with a corresponding relative humitidy not greater than 71 percent and the maximum temperature is 75 F (24 C) with a corresponding relative humidity not greater than 46 percent.
.
The area where surface cleaning of the laminates is done should be isolated from operations that generate dust, oil vapors, or other contaminants.
.
The operation of fume-producing equip-ment and fuel-powered combustion engines should not be allowed. Battery-powered equipment is permitted.
.
Machining or other dust-producing process-es should not be allowed in the repair area once the repair of the damaged structure has been initiated.
.
Protect all surrounding structures that may come in contact with adhesive prior to appi-cation procedures.
.
Material specification must be referenced for specific adhesive application requirements.

Cosmetic repair. Cirrus clas-sifies a cosmetic repair as that which is designed to repair local-ized surface defects to the original profile and to prevent UV damage and moisture ingress. Cosmetic repairs relate to minor defects which have no significant effect on the structural strength of the structure.
Minor repair. Cirrus defines minor and major repairs the same as the FARs do. So, a minor repair would be a repair that is not a major repair.
Major repair. A major repair is a repair, that if done improperly might appreciably affect weight, balance, structural strength, per-formance, powerplant operation, flight characteristics, or other qualities affecting airworthi-ness; or a repair that is not done according to accepted practices or cannot be done by elementary operations.
Cirrus allows major repairs in the field if the repair area does not fall into a no-repair zone, and it does not require non-standard repair procedures. In addition, field repair is allowed if the repair is specifically covered in the maintenance manual. All other major repairs need Cirrus Design engineering disposition.
Restricted. Any repairs that occur in no-repair zones as listed in the maintenance manual are restricted. The mechanic needs to contact Cirrus Design for disposition.
Once we have gotten to this point, we know whether or not a repair is allowed in the area we are looking at. We can then begin the repair process. There are too many possible repair scenarios to cover in the limited space here, but we can cover some essential tips when perform-ing the repair.
Repair environment
One thing you need to be aware of is the repair environ-ment. For cosmetic repairs, the environment is not as critical. However, Cirrus has stringent environment requirements for minor and major repairs.
From our own safety perspec-tive, we want to make sure the repair is carried out in an area that has adequate ventilation.
We also need to ensure that we are performing the repair in a controlled environment to ensure proper integrity of the repair and avoid any contamination issues. See the Repair Environment side-bar on this page for some guide-lines on proper composite repair environment.
Ply orientation
When looking at a repair, we need to know the ply lay-up. The zone diagrams in chapter 51 of the maintenance manual provide that information. The charts con-tain information such as ply count and warp clocks. They show the zero-degree reference for each section of the airframe.
Here is a tip for determining ply count. If you’re looking at a solid laminate and don’t know the ply count, you can measure the thick-ness to determine it. The thickness of one ply is generally between
0.008 and 0.010 inch. So, if you measure the total thickness of the laminate and divide by 0.009, you can determine the ply count.
Wet lay-up
Cirrus has established a wet lay-up composite repair process. No vacuum system is necessary. Volinkaty says that the repair pro-cedure was chosen so that shops were not forced to buy expensive curing equipment. Instead, the mechanic fabricates a tent to cre-ate an envelope around the repair area. A dryer is used with a ther-mostat to control the cure cycle of the repair.
18 July 2006 . Aircraft Maintenance Technology . www.AMTonline.com . www.AMTSociety.org
Airframe Technology
Cirrus training
Cirrus wanted mechanics in the field to be knowledgeable about proper repair procedures when working on Cirrus air-craft. The company has created several training opportuni-ties that can help increase a mechanic’s proficiency when it comes to inspecting and main-taining Cirrus aircraft.
Online training
Cirrus has developed an online training program called the SR Series AvMx course. It covers inspecting, repairing, troubleshooting, and maintaining Cirrus SR-series airplanes. The course features seven interactive lessons, each with several multi-step challenges, that simulate aircraft maintenance in real-world situations such as busy schedules, difficult customers, and unpredictable situations. This course is a prerequisite for anyone wishing to attend other Cirrus training.
“Our online program has sev-eral benefits,” says Doug Larson, Cirrus senior technical instruc-tor. “First of all, while the first section of the course deals with technical publications, they are referenced throughout all seven lessons. The mechanic becomes proficient with using the manu-als. Second, it allows us to do a lot of the initial training before a mechanic steps foot in the Duluth classroom.” As Larson shares, mechanics have a widely varying amount of knowledge and experience when it comes to composites. By covering the basics in an online course, every-one starts off on a level playing field when they step foot in the Duluth classroom.

Duluth training
Cirrus provides a one-week composite repair laboratory in composite inspection and repair.
www.AMTSociety.org . www.AMTonline.com . Aircraft Maintenance Technology . July 2006 19
Airframe Technology
Above: The Cirrus composite repair laboratory allows mechanics to practice repair techniques on actual composite structures and gain real-world experience before having to do it on an aircraft.
If you enjoy sitting in a classroom for hours on end, this course is not for you. After a few hours of review and a safety briefing, you are in the lab with your respira-tor, safety glasses, and Tyvek suit performing composite repairs. The composite repair lab has a large resource of composite pieces on hand to work on. That way, the student gets the opportunity to get his or her feet wet by working on the actual structure he would see in the field before doing it on an actual aircraft.
I had the opportunity to take both the online course and the composite repair laboratory. Having never worked on a Cirrus before, I came away with informa-tion that I could apply to repair or inspection situations in the field. These courses are a valuable resource for any mechanic who will be maintaining or inspecting Cirrus aircraft. AMT
Additional ReSource
Cirrus Design Corporation www.cirrusdesign.com

ever overs en .
ever overs en .
N
ever oversp
end
.
ever un erse .
ever un erse .

ever und
ersel
l
.
With a subscription to ILS, you re in touch with a market where prices
for goods and services are driven solely by supply and demand. That s
why, for over 15,000 subscribers worldwide, ILS is the market.
Inventory Locator Service,. LLC 1-800-233-3414 (North America) . 1-901-794-5000 (Worldwide) . 1-901-794-1760 (Fax) E-mail us at: ebusiness@ILSmart.com or see our website at www.ILSmart.com
20 July 2006 . Aircraft Maintenance Technology . www.AMTonline.com
. www.AMTSociety.org

fran001

学习学习，谢谢